1. Field of the Invention
The present invention relates to a circuit and a method for estimating a carrier frequency offset, and more specifically, to a band averaging circuit and a related method for estimating a carrier frequency offset in a multi-band and multi-carrier communication system.
2. Description of the Prior Art
In general, data to be transmitted is modulated by a carrier in a communication system to ensure the communication quality of a transmitter and a receiver. In the communication system, the data is modulated by the carrier of a specific frequency at the transmitter utilizing a local oscillator, the modulated data is transmitted through a channel, and the modulated data is received by the receiver. The receiver must utilize a local oscillator to demodulate the received data before reading. However, practically, even two identical oscillators cannot generate oscillating signals of completely the same frequency and therefore there is a slight difference between the original data and the demodulated data.
For example, a multi-band multi-carrier communication system transmits a packet using several different carriers, i.e. three carriers, and the packet received at the receiver must be respectively demodulated according to the frequencies of the three carriers to obtain the original packet. To ensure the frequencies of the carriers utilized at the transmitter and the receiver are identical, a carrier frequency offset estimation circuit is utilized for estimating and compensating the frequency offset between the carriers utilized by the transmitter and the receiver. Ideally, a ratio of a carrier frequency offset to a center frequency of the carrier is equal to a specific value, so most of the conventional methods estimate the ratio of the frequency offset of each carrier to the frequency of the each carrier to generate the first, second, and third carrier frequency offset ratios. The carrier frequency offset of the each carrier is then obtained according to an average value of the first, second and third carrier frequency offset ratios. The above-mentioned computational operation is in the following equation:
                              γ          avg                =                              1            3                    ·                      (                                                            Δ                  ⁢                                                                          ⁢                                                            f                      ^                                        1                                                                    f                                      c                    ⁢                                                                                  ⁢                    1                                                              +                                                Δ                  ⁢                                                                          ⁢                                                            f                      ^                                        2                                                                    f                                      c                    ⁢                                                                                  ⁢                    2                                                              +                                                Δ                  ⁢                                                                          ⁢                                                            f                      ^                                        3                                                                    f                                      c                    ⁢                                                                                  ⁢                    3                                                                        )                                              Equation        ⁢                                  ⁢                  (          1          )                    
γavg denotes the average carrier frequency offset ratio, fc1 to fc3 denote center frequencies of the first, the second and the third carriers, respectively, and Δ{circumflex over (f)}1, Δ{circumflex over (f)}2, Δ{circumflex over (f)}3 denote the carrier frequency offsets of the three carriers, respectively. Because estimation results of the carrier frequency offset ratios may not be accurate, it is necessary to average the carrier frequency offset ratios in order to obtain an average carrier frequency offset ratio γavg, which will better approximate the above-mentioned specific value. Then, a carrier frequency offset of each carrier is generated using the average carrier frequency offset ratio γavg. The above-mentioned computational operations for obtaining the carrier frequency offsets are provided in the following equation:Δ{tilde over (f)}1=γavg·fc1  Equation (2)Δ{tilde over (f)}2=γavg·fc2  Equation (3)Δ{tilde over (f)}3=γavg·fc3  Equation (4)
In equations (2) to (4), Δ{tilde over (f)}1, Δ{tilde over (f)}2 and Δ{tilde over (f)}3 denote carrier frequency offsets calculated according to the average carrier frequency offset ratio γavg, respectively, and the carrier frequency offsets Δ{tilde over (f)}1, Δ{tilde over (f)}2 and Δ{tilde over (f)}3 are utilized to compensate the local oscillating signal of the receiver.
By using the above-mentioned equation (1), a more accurate carrier frequency offset can be obtained. However, when estimating any of the first, the second and the third carrier frequency offset ratios, if a serious mistake occurs, the accuracy of the average carrier frequency offset ratio γavg will be affected seriously by the mistake accordingly.